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C E L L B I O L O G YP H D C O U R S E W O R K .
P R E S E N T E D B Y
JAHIR AHMED
Protein Transport, Targeting and Sorting.
10-9Folding in the Endoplasmic
reticulum
COP 2 TRANSPORT VESICLE.
Toc components, mediate
translocation (Toc75 is the
translocon); it is unclear how
preproteins are targeted to the
channel; Hsp70/Hsp40 may be
involved
Hsp70 in both the IMS and the
stroma assist the threading of the
preprotein into the chloroplast
an Hsp100 chaperone also called
ClpC (AAA ATPase) also binds
preproteins in the stroma
Hsp70/chaperonin (Cpn60) may
assist folding/assembly of newly-
imported protein
import into thylakoids (used for
respiration) uses the SRP pathway
10-9
Translocation into chloroplasts
targeting of proteins is initiated
post-translationally by Pex5/7
proteins, which bind the peroxisomal
targeting signal (PTS)
translocon not well defined;
possibility of vesicular budding?
gated pore that is regulated by
membrane proteins?
first organelle demonstrated to
import proteins without a PTS, by
virtue of assembly with other
proteins that contained a PTS
various protein oligomers are
imported into peroxisomes
antibodies with PTS, and 9 nm
gold particles could be imported
Other transport mechanisms likely involve
folded proteins, including the twin-arginine (Tat)
transport system of bacteria, and the cytoplasm-
to-vacuole targeting pathway of yeast
10-10
Translocation into peroxisomes
nuclear localization signal (NLS) is typically highly basic; e.g., the SV40 large tumor
antigen (T ag) has the sequence PKKKRKV
a/b1 importin hetero-dimer recognizes and binds the NLS (or b importin alone)
b importin docks with NPC and mediates interaction with Ran (GDP form)
directionality conferred by nature of guanine nucleotide bound to Ran
Ran binding protein (RanBP) is required for b importin binding to RanGDP; Ran
GTPase activating protein (RanGAP) and nucleotide-exchange factor (RCC) are
cytoplasmic and nuclear
cytopl. RanGDP required for import; nuclear RanGTP required for release
conversely, RanGTP binds substrate with NES in the export direction
proteins to be imported can be in a native/near native form
10-12
Import into the nucleus
some nuclear pore proteins (nucleoporins)
contain core FxFG repeats (yellow)
b importin contains ‘heat’ repeats that bind the
FxFG repeats (Heat repeats 5, 6, 7 are shown in
red, green and blue)
the FxFG repeats interdigitate in grooves formed
by the Heat repeats
interaction of b importin with nucleoporins allows
transport across the nuclear pore complex
Core FxFG repeats found in nucleoporins.
Each repeat is separated by a ‘linker’ region:
Bayliss et al. (2000) Cell 102, 99-108.
10-14
Mechanism of import into nucleus
Oligosaccharide processing in Golgi compartments
Protein Targeting in Golgi
Protein Secretion Pathway (E.R – Golgi and Vice Versa)
Thank You…
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